Change semaphores array for a structure

Change semaphores array for a structure

This change is mainly done because norminette sees the old declaration
of semaphores[sem_num] as VLA. Instead of changing the enum, it may be
better to reimplement semaphores as a structure.
This may make a little bit more sense syntactically also?
(compare semaphores[forks] with semaphores.forks)